The present invention relates to a sheet feeder apparatus, more specifically to an apparatus wherein by means of suction heads sheets are lifted from a stack in order to be supplied to a machine, for example, a printing press or the like.
It is known to drive the suction heads vertically up and down by means of control cams secured to a drive shaft, whereby the cams operate levers and tension springs for controlling the vertical movement and thus the lifting of sheets from a stack. Simultaneously, such levers keep the suction nozzles in a raised position until the trailing edge of a sheet taken over by feed advance suction heads has cleared the range or path of travel of the lifting suction heads.
German Pat. No. 2,132,438 first published on Jan. 18, 1973, German patent publication no. 1,929,714 published on Oct. 8, 1970 and East German Pat. No. 9,237 published Feb. 17, 1955 disclose various prior art sheet lifting and feed devices. Thus, suction heads for sheet feeders are known in which the sheet separating suction heads are controlled mechanically by rotating cam means and pneumatically by the application of suction air to the separating nozzles of these suction heads, whereby such application of suction air is accomplished in a timed sequence. A pivotable control lever bears against the cam track of a cam by means of a cam follower roller for accomplishing the mechanical control. A tension bar is secured to the free end of said control lever either in a pivotable or in a guided fashion. The tension bar in turn supports the separating suction heads and their carrier means.
A tension spring secured to the control lever assures that the cam follower roller of the control lever remains in cooperating contact with the cam track at least during the operation of the apparatus. The type of arrangement poses difficulties because the separating suction heads and their carrier means constitute a substantial mass, the inertia of which counteracts the effect of the tension spring.
Due to the fact that a substantial inertia tends to eliminate the action of the tension spring, the latter is subjected to large loads, especially where high sheet transport speeds are involved. Accordingly, the tension spring means are subject to substantial wear and tear and must be exchanged frequently. The just described prior art devices are substantially described in the above first mentioned two German patent publications. Further, a time delay results from the spring inertia, especially of heavy duty springs used in mechanical lifters.
The East German Pat. No. 9,237 discloses an apparatus for holding-up the separating suction nozzles of the suction heads by controlled pawl-type lever means. One such pawl-type lever means is provided for each separating suction nozzle. Each pawl-lever requires a substantial number of suitable translating lever means with the respective pivot or journal means which are subject to substantial wear and tear. Further, the suction heads disclosed in said East German patent require similarly as in the first mentioned two West German patent publications, mechanical means for the up and down movement of their separating suction nozzles. Such mechanical means are necessary in addition to the pneumatic actuation of the nozzles.
The above described features of the prior art especially the fact that substantial masses are involved in the movement of the suction means pose a limitation on the working speed of prior art devices which is not compatible with presently required sheet feed advance speeds for example in a high speed printing press.